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Mineral oil based in large part, yes. PCB's were used in conjunction with mineral oil till 1979, as they were very toxic when burned. There are new oils being developed, but not really yet out there in use as of yet.
I'm not at all certain how well mineral oil would do in a diesel engine. Having a spare diesel engine to run a test is as far as I can think of.
Kesephist
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I only brought this up to bring out the knowledge that it could be used as a last case scenario in the event of SHTF. I am in no way suggesting we all go out and cut down all transformers. And if SHTF happens, tractors will be a dime a dozen here in my area. It it burns up the engine, just go find another tractor.